The presence of cancer stem cells was suggested by the hypothesis that the proliferation of tumors is maintained by some cells having high malignancy among tumor cells. In fact, cancer stem cells have been isolated from blood cancers and solid cancers. Such cancer stem cells can generally form tumors, can induce the metastasis of tumors in vivo, and can create other types of cancer stem cells capable of differentiating into cells having various characters (Baccelli I et al., J Cell Biol 198:281-293, 2012). Thus, these cancer stem cells have outstanding characteristics in that they divide asymmetrically and have the ability to self-renew and proliferate to form tumors.
According to reports to date, it is a common theory that, due to the abilities of cancer stem cells to divide and metastasize, tumors cannot be removed only by induction of apoptosis of general cancer cells present in the tumors. Cancer stem cells in tumors are supported by microenvironments (niches) that can maintain the characteristics of these cells, and such environments including various immune cells, stromal cells, cancer cells and extracellular matrix may impart the characteristics of cancer stem cells to cells other than cancer stem cells. However, cancer stem cells remain in the resting state, unlike other cancer cells, and are less aggressive than metastatic cancer cells. Thus, cancer stem cells overcome the challenge of anticancer agents, and if a number of cancer stem cells present in tumors are activated due to external attack of inflammation or toxic substances, these cells are able to enter the cell cycle at high speed to thereby have the ability to proliferate (Wilson A et al., Nat Rev Cancer 7:834-346, 2007).
In preliminary experiments for the present invention, it was found that, when a cell population with high expression of CD44 that is one of cancer stem cells markers was isolated from metastasized breast cancer cells and subjected to a phosphorylation assay, the most highly phosphorylated protein was nuclear factor kappa B (hereinafter referred to as “NF-κB”). In fact, it is known that NF-κB is involved in the proliferation of cells and activated in cancer cells and cancer stem cells.
Generally, NF-κB of a p50/p65 in the resting state binds to IκB-α and exists in the cytoplasm in an inactive state (Simon, T. W. et al., Semin. Cancer Biol. 8:75-82, 1997). Because IκB-α bound to NF-κB masks the nuclear localization sequence (NLS) of NF-κB and IκB-α contains a strong nuclear export sequence (NES), NF-κB remains in the cytoplasm without moving to the nucleus (Thomas, H. et al., Cell. 68:1121-1133, 1992). If various intracellular and extracellular signals and stress are applied to cells, the degradation of IκB-α by protease due to its phosphorylation and ubiquitination will occur, and for this reason, the NLS of NF-κB masked by binding to IκB will be unmasked, whereby NF-κB remaining in the cytoplasm will be liberated, will rapidly move fast to the nucleus, and will bind to its target gene to activate the transcription of the gene.
It has been reported that the intracellular activation of NF-κB regulates the transcriptional activity of not only genes such as not only inflammation-related enzymes (cyclooxygenase II (COX-2), and inducible nitric oxide synthase (iNOS)), stress-response proteins, receptors (interleukin-2-receptor, and T-cell receptor), cytokines (interleukin-1, -2, -6 and -12, and TNF-α), and chemokines (interleukin-8), which are involved in inflammatory reactions, but also genes such as cyclin D1, TRAF1 (tumor necrosis factor-receptor associated factor 1), and c-IAP (inhibitor of apoptosis) 1, which are involved in apoptosis and cell proliferation (Barnes, P et al., Engl. J. Med. 366:1066-1071, 1997; Xie, Q et al., J. Biol. Chem. 269:4705-4708, 1994; Yamamoto, K. et al., J. Biol. Chem. 270:31315-31320, 1995).
In particular, it was found that abnormal NF-κB activation regulates various genes that are involved not only in inflammation-related diseases, but also in cell carcinogenesis, proliferation, invasion and angiogenesis (Aradhya, S. et al., Curr. Opin. Genet. Dev., 11:300-7, 2001; Orlowski, R. Z. et al., Trnds Mol. Med., 8:385-9, 2002). In addition, it has been reported that abnormal NF-κB activation plays an important role in malignancy of cancer cells. Furthermore, it is known that NF-κB confers drug resistance to cancer cells treated with anticancer agents and induces the expression of genes that promotes the survival of the cancer cells (Campbell K J et al., Mol Cell 13:853-865, 2004). As described above, it is expected that regulating the activity of NF-κB not only inhibits the growth of cancer stem cells, but also is involved in the signaling mechanism of cancer cells in microenvironments in which cancer stem cells exist, thereby exhibiting the effect of inhibiting tumor proliferation.
Accordingly, the present inventors have made extensive efforts to develop an effective anticancer therapeutic drug that inhibits the proliferation of cancer cells and cancer stem cells and that induces apoptosis of such cells. As a result, the present inventors have discovered and synthesized a functional peptide from an organic material present in vivo, and have found that, when breast cancer stem cells are treated with the peptide, the peptide exhibits the effect of inhibiting NF-κB activity to inhibit the proliferation of the cells and induce apoptosis of the cells, thereby preventing the present invention.